Why do some email attachments not appear

Question

Email with small pdf file attached had paper clip showing there is an attachment, but there is no attachment showing in the email. If I forwad the email, ipad asks if I want to include attachment. I say include and forward it to myself. I go to exchange email on my desktop computer and the email with attachment is there. Open the same email on ipad, shows paperclip, but there is no attachment to open. This by no means happens on all email with attachments, it happens on approximately 5% of email from all different sources. Most emails with attachments, even pdf's come through fine, but a small number do not show attachment even though it is there.

Answer 1

No similarities at all, in fact expanding on my description above:
     When I forward the email from my iPad(where i cannot see the attachment) to myself and then view it on my PC (which is running MS Office 2010), as I said I will see the attachment. I will then forward that to myself and open the email
on my iPad and there is no attachment (even though paper clip is there). Then I do the same exact thing from my PC, forward it to muself but this time I copy the same exact attachent into the forwarded email (so it has 2 attachments which are the same exact thing) and when I open that email on my iPad, I see one (1) attachment. I know this is very strange.